In recent years, with the spread of the internet, demand for international telecommunications to process large-volume contents such as voices and video images has been rapidly increasing. Accordingly, optical wavelength division multiplexing communication, where a plurality of optical signals of different wavelengths are simultaneously transmitted on a single optical fiber cable, has been widely used as a high-speed and large-capacity information communication means.
In particular, submarine cable systems are required to have very high reliability, because they are installed in deep sea and accordingly cannot be easily repaired. Because of this condition, research and development has been conducted on accuracy improvement and mutual interaction of a large number of relay devices intervening between the submarine cable systems.
FIG. 10 is a block diagram showing a configuration diagram of a typical optical branching/insertion device 700 existing between submarine cable systems. The optical branching/insertion device 700 comprises an optical coupler 200, an optical filter 400, an optical filter 500 and an optical coupler 600. The optical coupler 200 splits a trunk-side optical signal inputted from the outside and outputs the split signals respectively to the inside of the optical branching/insertion device 700 and to the brunch side. The optical filter 400 passes only a specific optical signal component (insertion signal) out of a brunch-side optical signal. The optical filter 500 passes only a specific optical signal component (pass-through signal) out of the trunk-side optical signal. The optical coupler 600 passes the specific optical signal component (insertion signal) out of the brunch-side optical signal, and combines the optical signal component thus passed with light outputted from the optical filter 500.
However, in the above-described configuration, if an input failure of the trunk-side optical signal occurs, no pass-through signal is inputted to the optical branching/insertion device 700. In that case, only an insertion signal is transmitted to the subsequent stage, and accordingly, there arises a problem of reduction in the total power. If an input failure of the brunch-side optical signal occurs, no insertion signal is inputted to the optical branching/insertion device 700. In that case, only a pass-through signal is transmitted to the subsequent stage, and accordingly, there also arises a problem of reduction in the total power.
In terms of the problem described above, for example, PTL 1 and PTL 2 each describe a technology for making the total power of output light having been propagated through an optical fiber equivalent to that of the incident light.
In the technology of PTL 1, a total sum of powers of an optical signal inputted from a trunk line and that from a brunch line is compared with a threshold value set in advance. On the basis of the comparison result, the power of the input optical signal from the trunk line is controlled.
In the technology of PTL 2, when an optical input at the front stage of the device becomes in a no-input state owing to a transmission line failure or the like, inputted spontaneous emission light is adjusted to have the same level of output power as that of a pass-through optical signal in the ordinary state, and the adjusted light is outputted as compensation light.